1. Field of the Invention
The present invention relates to an ink tank and a recording apparatus using the ink tank.
2. Description of the Related Art
Examples of recording apparatuses using ink contained in an ink tank include inkjet recording apparatuses having an inkjet recording head capable of ejecting ink. Such inkjet recording apparatuses include a serial-scanning-type inkjet recording apparatus that records an image on a recording medium by moving a carriage including an inkjet recording head and an ink tank in a main scanning direction.
The serial-scanning-type inkjet recording apparatus includes a carriage having an inkjet recording head and an ink tank for supplying ink to the inkjet recording head. To perform recording, the inkjet recording apparatus moves the carriage relative to a recording medium and ejects ink droplets from a small ejection port. The ejected ink droplets are deposited on the recording medium so that a desired image is recorded.
Typically, dye ink has been used as ink for the inkjet recording head. The dye ink includes dye as a colorant. However, it is difficult to maintain the quality of dye ink for outdoor printed materials (such as resistance to light and resistance to climate). Therefore, instead of dye ink, pigment ink that includes a pigment as a colorant has been developed. However, since the pigment is not dissoluble, but dispersible, it is inevitable that pigment particles settle out inside an ink tank.
For example, if an ink tank is mounted in an inkjet recording apparatus and the inkjet recording apparatus is not operated for a long time, pigment particles in ink gradually settle out inside the ink tank. Accordingly, in the ink tank, a pigment dispersion graded in concentration from the bottom to the top of the tank occurs. As a result, the concentration of pigment particles in the lower portion of the ink tank is high so that an ink layer having too dark a color is formed. In contrast, in the upper portion of the ink tank, the concentration of pigment particles is low so that an ink layer having too light a color is formed.
When the ink in the ink tank is retrieved from the bottom of the tank and the retrieved ink is supplied to the recording head, the ink in the layer having high concentration of pigment particles is supplied first. Thus, an image having too dark color is recorded. That is, an easily recognizable difference may be created between the recording density of an image formed in an initial stage of use and that in a late stage of use. In particular, such a difference is noticeable in color recording for recording a color image using different densities of colors.
Japanese Patent Laid-Open No. 2004-216761 and Japanese Patent Laid-Open No. 2005-066520 describe a structure in which an ink tank includes a stirrer which is moved by means of an inertia force caused by the reciprocal movement of a carriage. Thus, ink is agitated in the ink tank.
That is, Japanese Patent Laid-Open No. 2004-216761 describes an ink tank including a freely swingable stirrer. The pivot of the swinging movement of the stirrer is determined to be positioned at substantially the center of the ink tank in the direction of the carriage movement. Thus, the stirrer moves similarly in one direction and the opposite direction by means of the reciprocal movement of the carriage. Additionally, Japanese Patent Laid-Open No. 2005-066520 describes an ink tank including a freely swingable stirrer that resiliently deforms. This stirrer hangs from substantially the center of the inner top surface of the ink tank. Thus, the stirrer also provides similar movements in one direction and the opposite direction by means of the reciprocal movement of the carriage. Furthermore, Japanese Patent Laid-Open No. 2005-066520 describes a structure in which an ink tank includes a stirrer that is freely movable on the bottom surface of the ink tank. Thus, the stirrer can freely move on the bottom surface of the ink tank in accordance with the reciprocal movement of the carriage.
However, the ink tanks described in Japanese Patent Laid-Open No. 2004-216761 and Japanese Patent Laid-Open No. 2005-066520 have the following disadvantages.
First, in the ink tank described in Japanese Patent Laid-Open No. 2004-216761, the stirrer swings about substantially the center of the ink tank similarly in one direction and the opposite direction. Accordingly, in order to increase the agitating performance by increasing the swing range of the stirrer, the width of the ink tank needs to be increased in the carriage movement direction. However, since, in most cases, a plurality of ink tanks are mounted in the carriage along the carriage movement direction, the width of the ink tank is limited to a relatively small value. Thus, the swing range of the stirrer cannot be increased. Therefore, the ink flow caused by the swing of the stirrer is weak. To sufficiently agitate the ink, the agitating time needs to be increased by increasing the number of reciprocal movements of the carriage.
In contrast, in the ink tank described in Japanese Patent Laid-Open No. 2005-066520, a stirrer hangs from substantially the center of the inner top surface of the ink tank and swings about substantially the center of the ink tank similarly in one direction and the opposite direction. To increase the agitating performance by increasing the swing range of the stirrer, the width of the ink tank needs to be increased in the carriage moving direction, like the ink tank described in Japanese Patent Laid-Open No. 2004-216761. Therefore, this structure has the same disadvantage as that of the ink tank described in Japanese Patent Laid-Open No. 2004-216761. Additionally, if the acceleration of the carriage is set to be high in order to cause large elastic deformation of the stirrer, the size and the manufacturing cost of a driving motor for driving the carriage increases. In addition, the vibration of the recording apparatus may increase. Furthermore, for the ink tank described in Japanese Patent Laid-Open No. 2005-066520 and having a stirrer freely movable on the bottom surface of the ink tank, since the stirrer is distant from the upper layer of ink in the ink tank, the performance of agitating the ink in the upper layer is not satisfactory.
Such disadvantages described in Japanese Patent Laid-Open No. 2004-216761 and Japanese Patent Laid-Open No. 2005-066520 are easily predictable from the following structure of widely used ink tanks and recording apparatuses.
In general, an ink tank mounted in a carriage (hereinafter also referred to as an “on-carriage-type ink tank”) has a width and a length so that the operation of removing and attaching the ink tank from and to the carriage is easily carried out. That is, the width of the ink tank in the carriage moving direction is relatively small and the length of the ink tank in a moving direction of a recording medium (in the sub-scanning direction, which is perpendicular to the main scanning direction) is relatively large. Therefore, in the main scanning direction, which is a direction in which the stirrer is displaced, a large amount of displacement cannot be achieved. Since the amount of displacement of the stirrer is small, a strong ink flow cannot be generated. For this reason, the efficiency of agitating the ink is low and a long time is required for agitating the entire ink inside the ink tank. For example, when the recording apparatus with an ink tank mounted to the carriage is not operated for a long time and pigment particles of the ink in the ink tank settle out, reciprocal movements of the carriage is required for a long time before the recording operation starts. Thus, a long warm-up time is required before the recording operation becomes ready. In particular, when the diameter of the pigment particles of the pigment ink is large or a specific gravity of the pigment particles is large, the settlement is rapid. If the ink tank remains unused even for several days, the concentration distribution of ink that causes an adverse effect on the quality of a recording image may be generated. In this case, the ink must be agitated every few days. In addition, during the agitating operation, recording operation cannot be carried out.